The prior processes using wheat gluten as a fermenting material are disclosed in Japanese Patent Kokai 50-71900, 51-38496, 54-37894 and 62-239965, which are all directed to the treatment of wheat gluten by an added water-steaming process as is done for soybean. However, added water provides wheat gluten with a specific viscoelasticity not observed in other materials, which makes it troublesome to operate a thermal denaturation subsequent to water addition. In "Journal of Japan Soy Sauce Research Institute, vol. 4, No. 6, pp. 227-232, Nov. 25, 1978" issued by Japan Soy Sauce Research Institute, it is reported therein that koji prepared by adding water to wheat gluten, steaming, mixing with steamed rice and cultivating a koji mold therein, will form a mass which presents a problem in the development of a koji mold into gluten.
Accordingly, it is conventional that the above problem has been dealt with by a process of adding small amounts of wheat gluten to the surface of the water-added soybean and steaming or a process of adding gluten to moromi without a koji-making.
However, the process of adding wheat gluten to moromi has the problem that addition of insufficiently denaturated gluten to moromi makes the enzymatic digestion property very poor and causes insufficient denaturation of protein.
As the technique to apply gluten for koji-making, Japanese Patent Kokai 50-94195 discloses a process of adding water to a mixture of a starchy material powder and wheat gluten to form a dough, steaming and granulating the dough to give a koji-making material. However, this process involves the problems in which (i) steaming the mixture in the form of a dough mass may provide poor thermal conductivity, non-uniformity in thermal denaturation and a reduced total nitrogen utilization ratio, (ii) formation of dough by kneading the mixture with added water may produce a highly viscous product which, even when granulated, will form a mass in koji-making to putrefy koji, (iii) the granulated product after dough formation has a sheet-like surface, which results in no development of koji mold into the dough, thus reducing in the enzymatic activity of koji and no extaction of flavor enhancers.